In the colonial ascidian Botryllus schlosseri, tributyltin (TBT), a powerful antifouling biocide, acts as immunotoxic xenobiotic since, at a sublethal concentration (10 µM), it causes an irreversible and significant decrease in in vitro yeast phagocytosis, associated with considerable changes in the shape of phagocytes, which withdraw their pseudopodia and become spherical, due to structural damage of cytoskeletal components. The addition of TBT to the culture medium causes a significant decrease in the amoebocytic index, i.e. the percentage of amoeboid-shaped haemocytes, and prolonged washing in sea water never succeeds in restoring amoeboid shape. In these cytoskeletal alterations, F-actin undergoes extensive depolymerisation, resulting in the absence of FITC-phalloidin fluorescence. Microtubules are not recognisable as single filaments with anti-alpha-tubulin immunofluorescence, although the centrosome is not affected. The addition of increasing exogenous calmodulin (CaM) concentrations (from 20 to 120 µM) after incubation in TBT determines a significant increase in the amoebocytic index, although it is not able to bring it to that of controls, suggesting that CaM in the medium in any case externally exerts an influence on haemocytes pretreated with TBT. The copresence of TBT and exogenous CaM at concentrations higher than 80 µg/ml restores the amoebocytic index and cytoskeletal morphology. The latter appears complete for microtubules and partial for microfilaments. Experiments with isodynamic mixtures of TBT and specific CaM inhibitors, i.e. chlorpromazine (CPZ) and N-(6-aminohexyl)-5-chloronaphtalene-1-sulfonamide (W-7), reveal the synergistic effect of antagonism, indicating competition for the same site - a Ca(2+)-CaM hydrophobic region - by both interacting substances and, therefore, the formation of a TBT-CaM complex. Instead, isodynamic mixtures with thapsigargin, an inhibitor of Ca(2+)-ATPase of the endoplasmic reticulum, have an effect of potentiation, suggesting that TBT indirectly interacts with this Ca(2+)-ATPase activity. We hypothesise that the main mechanism of action of TBT in B. schlosseri phagocytes is alteration of Ca(2+) homeostasis by means of direct interaction with endogenous CaM, which induces a conformational change preventing the regulative activity of CaM on Ca(2+)-ATPase. Consequently, an excess of cytosolic Ca(2+) accumulates which, together with the inhibition of CaM-dependent kinases and Ca(2+)-regulated proteins, produces extensive cytoskeletal disorganisation.